1. Field of the Invention
The present invention relates to a method and apparatus for measuring the waveform quality of a CDMA (Code Division Multiple Access) modulated signal that is used in mobile communications or the like.
2. Description of the Related Art
A conventional method for measuring evaluation parameters of each channel signal in a radio wave radiated from a base station of CDMA mobile communications, that is, a power coefficient .rho..sub.i, the output timing .DELTA..tau..sub.i and a phase offset .DELTA..theta..sub.i, is described, for example, in a thesis entitled "Overview of Code-Domain Power, Timing, and Phase Measurements", Hewlett-Packard Journal, pp.73-93, February 1996.
Referring to FIG. 3, the conventional method will be described in brief. A CDMA signal from a base station is inputted via an input terminal 11 into a down converter 12. The CDMA signal is converted by the down converter 12 to an IF signal which is, in turn, amplified by an amplifier 13. The amplified IF signal is band-limited by a filter 14 and converted by an A/D (analog-to-digital) converter 15 to a digital signal. The measuring signal (the CDMA signal), converted to digital form, is transformed by a quadrature transform part 16 to a complex base band signal. The base band signal is applied to a frequency/phase compensating part 17 wherein its frequency and phase shifts are compensated for. The compensated base band signal is applied to a .rho..sub.i computation/bit detection part 18 and a parameter evaluating part 19.
A pilot signal generating part 20 is a means for generating a known pilot signal. Based on the known pilot signal from the pilot signal generating part 20 and the output from the frequency/phase compensating part 17, the .rho..sub.i computation/bit detection part 18 computes the power coefficient .rho..sub.i and detects a bit for each channel. The power coefficient .rho..sub.i and the bit thus detected are provided to an ideal signal generating part 21 which generates an ideal signal from the detected bit and power coefficient .rho..sub.i and the pilot signal fed from the pilot signal generating part 20. The ideal signal is applied to the parameter evaluating part 19.
The parameter evaluating part 19 is also supplied with the output from the frequency/phase compensating part 17. Based on the both input signals, the parameter evaluating part 19 computes an output timing (a time lag) .DELTA..tau..sub.i and a phase offset (a phase shift) .DELTA..theta..sub.i of each of other channels relative to a pilot channel in the input signal (the CDMA signal) so as to minimize the mean square of the difference between the two input signals. The thus computed outputs .DELTA..tau..sub.i and .DELTA..theta..sub.i are fed back to the ideal signal generating part 21. The ideal signal generating part 21 re-creates and applies an ideal signal to the parameter evaluating part 19 which repeatedly computes the output timing .DELTA..tau..sub.i and the phase offset .DELTA..theta..sub.i.
The above conventional method computes the output timing .DELTA..tau..sub.i and the phase offset .DELTA..theta..sub.i in the parameter evaluating part 19 through their optimization but does not involve optimization of the power coefficient .rho..sub.i. On this account, the computed power coefficient .rho..sub.i remains affected by the output timing .DELTA..tau..sub.i and the phase offset .DELTA..theta..sub.1, and hence its accuracy of measurement is low. Since the ideal signal generating part 21 uses the low-accuracy power coefficient .rho..sub.i to generate the ideal signal, the measured output timing and phase offset .DELTA..tau..sub.i and .DELTA..theta..sub.i also lack accuracy.